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<H3>improper_style command 
</H3>
<P><B>Syntax:</B>
</P>
<PRE>improper_style style 
</PRE>
<UL><LI>style = <I>none</I> or <I>hybrid</I> or <I>class2</I> or <I>cvff</I> or <I>harmonic</I> 
</UL>
<P><B>Examples:</B>
</P>
<PRE>improper_style harmonic
improper_style cvff
improper_style hybrid cvff harmonic 
</PRE>
<P><B>Description:</B>
</P>
<P>Set the formula(s) LAMMPS uses to compute improper interactions
between quadruplets of atoms, which remain in force for the duration
of the simulation.  The list of improper quadruplets is read in by a
<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A> command
from a data or restart file.  Note that the ordering of the 4 atoms in
an improper quadruplet determines the the definition of the improper
angle used in the formula for each style.  See the doc pages of
individual styles for details.
</P>
<P>Hybrid models where impropers are computed using different improper
potentials can be setup using the <I>hybrid</I> improper style.
</P>
<P>The coefficients associated with an improper style can be specified in
a data or restart file or via the <A HREF = "improper_coeff.html">improper_coeff</A>
command.
</P>
<P>All improper potentials store their coefficient data in binary restart
files which means improper_style and
<A HREF = "improper_coeff.html">improper_coeff</A> commands do not need to be
re-specified in an input script that restarts a simulation.  See the
<A HREF = "read_restart.html">read_restart</A> command for details on how to do
this.  The one exception is that improper_style <I>hybrid</I> only stores
the list of sub-styles in the restart file; improper coefficients need
to be re-specified.
</P>
<P>IMPORTANT NOTE: When both an improper and pair style is defined, the
<A HREF = "special_bonds.html">special_bonds</A> command often needs to be used to
turn off (or weight) the pairwise interaction that would otherwise
exist between a group of 4 bonded atoms.
</P>
<HR>

<P>Here is an alphabetic list of improper styles defined in LAMMPS.
Click on the style to display the formula it computes and coefficients
specified by the associated <A HREF = "improper_coeff.html">improper_coeff</A>
command.
</P>
<P>Note that there are also additional improper styles submitted by users
which are included in the LAMMPS distribution.  The list of these with
links to the individual styles are given in the improper section of
<A HREF = "Section_commands.html#cmd_5">this page</A>.
</P>
<UL><LI><A HREF = "improper_none.html">improper_style none</A> - turn off improper interactions
<LI><A HREF = "improper_hybrid.html">improper_style hybrid</A> - define multiple styles of improper interactions 
</UL>
<UL><LI><A HREF = "improper_class2.html">improper_style class2</A> - COMPASS (class 2) improper
<LI><A HREF = "improper_cvff.html">improper_style cvff</A> - CVFF improper
<LI><A HREF = "improper_harmonic.html">improper_style harmonic</A> - harmonic improper
<LI><A HREF = "improper_umbrella.html">improper_style umbrella</A> - DREIDING improper 
</UL>
<HR>

<P><B>Restrictions:</B>
</P>
<P>Improper styles can only be set for atom_style choices that allow
impropers to be defined.
</P>
<P>Most improper styles are part of the MOLECULAR package.  They are only
enabled if LAMMPS was built with that package.  See the <A HREF = "Section_start.html#start_3">Making
LAMMPS</A> section for more info on packages.
The doc pages for individual improper potentials tell if it is part of
a package.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "improper_coeff.html">improper_coeff</A>
</P>
<P><B>Default:</B>
</P>
<PRE>improper_style none 
</PRE>
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